Sahih Al-Bukhari — Hadith #6890

"When it was the day of (the battle of) Uhud, the pagans were defeated. Then Satan shouted, "O Allah's worshipers! Beware of what is behind you!" So the front files attacked the back files of the army. Hudhaifa looked, and behold, there was his father, Al-Yaman (being attacked) ! He shouted (to his companions), "O Allah's worshipers, my father, my father!" But by Allah, they did not stop till they killed him (i.e., Hudhaifa's father). Hudhaifa said, "May Allah forgive you." (`Urwa said, Hudhaifa continued asking Allah's Forgiveness for the killer of his father till he died.